Several processes for desulfurization of flue gas from combustion of fossil fuels utilize an absorbent based on a basic calcium compound such as hydrated lime, quick lime and lime stone.
Contact between the flue gas to be desulfurized and the absorbent may be provided as a gas-solid contact (dry process), as a scrubbing of gas by an aqueous suspension of the absorbent (wet process) or as a spray drying-absorption process (SDA-process) in which a suspension of the absorbent is spray dried in the gas, or by a combination of such contact methods. In the dry process and the SDA-process the spent absorbent is purged as a dry powder while the wet process delivers a sludge. In case the flue gas to be desulfurized containsfly ash the spent absorbent will have a substantial fly ash content. The calcium will mainly be present as sulfite and sulfate and, especially when the flue gas originates from the combustion of coal, a certain amount of calcium chloride will be present, typically between a half and a couple of weight percent CaCl.sub.2. In case the absorbent has been used for treating flue gas originating from the incineration of waste materials comprising e.g. chlorine-containing plastics, the amount of calcium chloride in the spent absorbent may be substantially higher.
In connection with wet desulfurization processes it is conventional to transform the calcium sulfite into calcium sulfate by oxidation by means of air in aqueous medium and to reduce the chloride content of the resulting material by washing with water. However, disposal of the resulting chloride containing solution may be restricted by legislation. A method for reducing the chloride content of the spent absorbent from the wet desulfurization without creating a chloride solution, would therefore be needed.
It has turned out that also the sulfite in the spent absorbent resulting from the dry process or the SDA-process may be oxidized conveniently, viz by heating the spent absorbent in the presence of oxygen at a temperature of 350.degree.-600.degree. C.
The oxidized spent absorbent is suitable for use as a calcium sulfate anhydrite material in cement manufacture or for other purposes in which calcium sulfate anhydrite is conventionally applied, e.g. for Estricht cement, even when fly ash is present in substantial amounts. For said uses it is, however, essential that the chloride content is reduced to a low level.
It has turned out that chloride may be removed by heating the spent absorbent at a temperature between 150.degree. and 600.degree. C. in the presence of a material which with chloride forms salts susceptible to hydrothermal decomposition, resulting in formation of HCl, or evaporation. Said methods, however require addition of e.g. magnesium, iron or ammonium compounds which increases the costs and complicates the process and may involve formation of harmful residues.